In the fabrication of an integrated circuit, there are steps during which a wafer rests on a moveable stage under a projector. As the stage moves, it experiences linear translation in a direction parallel to one of two coordinate axes that define the plane of the wafer. In addition, the stage experiences rotation about any of the three coordinate axes. During these steps, it is desirable to know, with great precision, the position and orientation of the moveable stage relative to the projector.
One approach to determining the motion of the stage is to use an interferometer to determine the distance to a measurement spot along the edge of the stage. This, however, only provides the extent to which the stage has translated along one of the coordinate axes. It doles not provide information on the rotation of the stage about the three coordinate axes.
To obtain information about both the rotation and the translation of the stage, it is known to use three interferometers to measure the distance to three different reference spots that define a triangle on the edge of the stage. From these three distance measurements, one can infer the extent and direction of rotation experienced by the stage. For example, if a first reference spot is found to have moved closer to the interferometer and a second reference spot immediately below the first spot is found to have moved further from the interferometer, one can infer that the plate has rotated about a line joining the first and second reference spots. From the relative positions and distances to the first and second reference spots, one can calculate the extent of this rotation.
It is also known to integrate the three separate interferometers into a single “multi-axis” interferometer, in which a single laser beam is split into three beams (one for each axis) by a beam multiplexer. These beams are directed to separate assemblies that typically include a polarizing beam splitter cube, a plane mirror, a corner cube retro-reflector, quarter-wave retarders, and mixing polarizers, all of which are oriented and configured to form reference beams and measurement beams for each interferometer axis. Assemblies of this type are described in U.S. Pat. No. 4,802,764 to Young et al. and U.S. Pat. No. 4,883,357 to Zanoni, et al., the contents of which are herein incorporated by reference.
The optical elements for directing the various beams in a multi-axis interferometer must be precisely aligned with each other for the multi-axis interferometer to provide accurate measurements. This alignment process is carried out at the time of installation and periodically thereafter, as the accumulated effects of continued operation cause a deterioration in alignment.
In some cases, it is desirable to measure the position of several structures using a single multi-axis interferometer. When this is the case, the number of interferometer axes, and hence the number of components to be aligned, increases. This makes alignment more difficult.